1. Field of the Invention
The present invention relates to a method for protecting a joint for electrical cables, in particular for underground electrical cables, to a protective coating intended to preserve the integrity of said joint once installed and to a joint for electrical cables thus protected.
2. Description of the Related Art
Generally cables for conveying or supplying energy, in particular for conveying or supplying medium-voltage or high-voltage energy, comprise, from the inside towards the outside of the cable: a metal conductor, an inner semiconductive layer, an insulating layer, an outer semiconductive layer, a metal screen—usually made of aluminium, lead or copper—and an external polymeric sheath. The predetermined sequence: metal conductor, inner semiconductive layer, insulating layer and outer semiconductive layer is generally known by the term “cable core”.
In order to form a joint between two sections of electrical cable, for example of the single-pole type, the ends of both said sections are processed beforehand so as to expose, over a portion of defined length, each of the aforementioned elements which make up the abovementioned cables.
Subsequently, the joining operation consists in forming an electrical connection between the conductor elements, for example by means of soldering or scarfing of the latter, and positioning, in the zone where the said conductor elements are joined together, an elastomeric sleeve conventionally known by the term “joint”.
Generally, said sleeve has a form which is substantially cylindrical in its central portion and of frustoconical type at its ends so as to provide an optimum connection between the cable sections being joined and the joint itself.
This sleeve consists of a plurality of radially superimposed elements intended to restore the electrical and mechanical connection between each exposed layer of a first cable section and the corresponding exposed layers of a second cable section.
Therefore, starting from its innermost portion, said sleeve generally comprises: a voltage distribution layer made of material with a high dielectric constant next to the insulating layers of the cable, a layer of insulating material of considerable thickness which surrounds said voltage distribution layer, and a layer of semiconductive material located radially on the outside of said insulating layer and suitably connected to the outer semiconductive layer of each cable section designed to restore the continuity of the outer semiconductive layers of said first and second section. Generally, the zone where the two conductor elements are joined together is filled with an electrical-field control material.
Methods for constructing joints known in the art are described, for example, in documents EP-379,056; EP-393,495; EP-415,082; EP-199,742; EP-422,567 in the name of the Applicant.
Generally, this sleeve is produced separately and supplied fitted, in an elastically-dilated condition, on a hollow tubular support made of rigid plastic. The sleeve thus supported is engaged around one of the sections during a step preceding the formation of the joint between the metal conductors.
This support may be constructed using different methods which allow the removal thereof once the abovementioned joint has been formed. For example, the tubular support may be obtained from a strip-like element helically wound so as to form a plurality of adjacent spirals fastened together so that, when a pulling force is exerted on a free end portion of said strip-like element, the tubular support is able to collapse, due to gradual separation of the spirals, and allow correct positioning of the sleeve. In so doing, the sleeve elastically contracts, gripping the cable sections in the joining zone. This sleeve is of the cold-retractable type. Embodiments of said supports are described, for example, in the documents EP-541,000; EP-735,639; EP547,656; EP-547,667 in the name of the Applicant.
Alternatively, the sleeves may be made using heat-shrinkable materials, thus producing the so-called heat-shrinkable sleeves described, for example, in the patent U.S. Pat. No. 4,383,131.
Generally a joint also comprises an element intended to restore the metal screen, such as, for example, a tin-plated copper strip which is applied starting from the exposed metal screen portion of the first section and terminating on the exposed metal screen of the second section.
In the case where the joining operation is performed between two sections of electrical cable of the multi-pole—for example double-pole or triple-pole-type, the procedure described hitherto is repeated for each single phase of each cable.
Finally, a joint as defined further above normally also comprises an external polymeric sheath suitable for restoring the external mechanical protection of the cable and fitted in the joining zone, in a position radially on the outside of the aforementioned sleeve.
Generally, this sleeve is intended to protect the underlying elements of the joint from coming into contact with moisture and/or water from the outside.
Said sheath may be of the heat-shrinkable type or cold-shrinkable elastic type or may be obtained by means of a strip-forming step, which may also be combined with the use of suitable mastic sealants.
This sheath is inserted on one end of one of the said cable portions during a step preceding both positioning of the tubular support carrying the abovementioned sleeve and formation of the connection between the conductor elements.
In accordance with further operating methods, restoration of the external mechanical protection of the cable may also be achieved using several sheaths, for example three in number, arranged so that one pair of sheaths is fitted onto the aforementioned frustoconical portions of said joint and a further sheath is fitted onto the substantially cylindrical portion of the latter.
Generally, the zone where two cables for conveying or supplying electric energy are joined together inevitably forms a discontinuity in the conveying or supply network and, consequently, a weak point in the latter, also in view of the complexity of the aforementioned joining zone.
This complexity is due, in fact, both to the plurality of operations which must be carried out by the technical personnel responsible for installation of the joint and to the structure itself of the joint in that its composition, as regards its main components, is as described further above.
In order to ensure a high degree of mechanical protection, guaranteeing optimum and long-lasting operation, the joints are generally provided on the outside of their structure with a protective coating having a suitable form and made of suitable materials, which enclose the joining zone internally.
It must be emphasized, in fact, particularly if the cables are positioned, as in most cases, in trenches dug in the ground, that inevitably the joints themselves also have to be arranged in position and made operative inside the said trenches.
However, the latter represent an environment which is difficult to control since, owing to their nature they have restricted dimensions, accumulated debris is present along the edges of the excavations and the technicians preparing the joint move around and operate within them.
Under such working conditions it frequently happens that the debris and/or work equipment used by said technicians may accidentally strike the external surface of the joints and cause, for example, deformations in the layer of insulating material forming part of the latter.
These deformations are particularly undesirable since they cause a reduction in the insulating capacity of said layer, as well as separation of the latter from the semiconductive layer, thereby giving rise to partial discharges, resulting in irreversible damage to the joint.
Known systems for protecting the joints from the environment surrounding them, in particular from dust and moisture, envisage, for example, the use of particularly simple containers which use rapid-closure systems, for example of the bayonet type as described, for example, in the patent U.S. Pat. No. 4,684,764.
Devices known in the art and designed to provide the joints with protection of the mechanical type, for example against accidental knocks which, as mentioned, may occur during laying and/or installation, consist, for example, of rigid containers positioned outside the said joints.
Generally, said containers are divided into two halves which are formed so as to be arranged around the joining zone and provide it with the desired protection. Moreover, they are often made of metallic material, for example aluminium, coated externally with an anti-corrosive paint.
Said paint has the function of avoiding, or at least limiting, the development of any corrosive phenomena which, locally deteriorating the external surface of said containers, in addition to weakening the mechanical strength thereof, would allow the undesirable infiltration of moisture and/or water inside the joint.
Generally, these containers have dimensions greater than those of the joint to be protected since, on the one hand, it is unfeasible from an economic point of view to produce containers with specific dimensions for each type of joint and cable to be joined and, on the other hand, during assembly, it is necessary to ensure that there is sufficiently ample room for manoeuvre to perform correct and rapid positioning of the protective container on the said joint.
Generally, a filling material is introduced inside said containers, namely into the gap between the external surface of the joints and the internal walls of the containers, said filling material performing the function of providing a protective layer against any accidental knocks affecting the joint and providing the joint protection system with a greater mechanical strength. If necessary, said filling material is also chosen so as to form a barrier against the infiltration of moisture and/or water from the outside.
Generally, the filling material which is used is a thermosetting resin such as, for example, an epoxy, polyurethane or similar resin.
The document GB-1,497,051 describes a further mechanical protection device for cable joints, consisting of a heat-shrinkable elastomeric sleeve, the internal surface of which is coated with a plurality of reinforcing elements of elongated shape, arranged parallel to the longitudinal axis of said sleeve.
Said reinforcing elements are generally in the form of wires, bars or strips of metallic, plastic or fibreglass material, which are kept in position adjacent to each other, for example by means of an adhesive, a mastic or a support sheet.
The document EP-093,617 relates to a further mechanical protection device for electrical cable joints, comprising a set of elongated elements which are kept adjacent to each other on the external surface of the joint and a hot-shreankable or cold-shreankable sleeve designed to be positioned around said set.
These elements, which are preferably made of metallic material, are fastened to each other so as to form a kind of cage structure by using, for example, cords, hooks, soldering zones, support sheets provided with an adhesive element or flexible strips.
Owing to the presence of said plurality of elements, this assembly is able to follow the profile of the joint where there are changes in its cross-section, reducing the overall dimensions of the coupling between protection device and joint.
In order to ensure that such a result may be achieved, each of the said elements is formed so as to have a substantially straight progression along the substantially cylindrical portions of the underlying joint and a diverging or converging progression where the cross-section of the joint respectively becomes thicker or thinner.
The abovementioned shreankable sleeve, which may also not be present, generally has a longitudinal extension greater than that of said elements so that the sleeve may make contact with a cable portion upstream of the joint and a cable portion downstream of the joint, sealing off the latter from the surrounding environment.
The Applicant has noticed that the protection devices for joints according to the known art have a plurality of drawbacks.
For example, in order to ensure a satisfactory mechanical impact strength, in the case where said device is in the form of a container located on the outside of the joint, generally this container is made of a material sufficiently rigid to safeguard the joint contained therein, for example metal or plastic material.
However, this feature is viewed as being particularly unfavourable since this material, being rigid, does not allow damping of an impact resulting from, for example, excavation debris falling inside the trench where the cable is laid, as the energy contained in said debris is transferred practically entirely onto the underlying joint.
Moreover, if this impact is particularly violent, it may cause permanent deformation of the protective container, resulting in it being ineffective in the damaged zone in the event of any renewed accidental impacts in the same zone and resulting in continuous crushing of the elements of the joint, adversely affecting the operation thereof.
As mentioned further above, in the case where the protective container is of the metallic type, it is generally coated with an anti-corrosive paint so as to prevent the occurrence of corrosive phenomena due to attack by water and/or moisture inevitably present in the ground.
However, the use of this anti-corrosive paint does not eliminate entirely this risk since any debris falling onto the external surface of the protective container inevitably forms incisions on the latter, even of a limited nature, resulting in removal of the paint.
These zones, therefore, constitute areas favouring the development of corrosion which, in particularly favourable environmental conditions, may develop rapidly and adversely affect the protective capacity of the container.
Containers of the metallic type do not have, moreover, any flexibility in the longitudinal direction, this aspect making installation thereof in an operative condition less easy.
In order to prevent water and/or moisture from spreading towards the joint to be protected, as mentioned further above, some solutions of the known art envisage using a filling material to be positioned in the gap between the container and the external surface of the joint. However, the use of this filling material has a few drawbacks.
A first drawback consists in the complexity of the protective system since the installation process envisages a first step for arranging the container around the joint and a next step during which the filling material is introduced into the space between joint and container and said material is left to harden or made to harden for example by using heat.
This means, therefore, that the operation involving preparation of the protective coating of the joint is fairly complex and requires a fairly long assembly time and the use of qualified technical personnel—these aspects obviously resulting in a substantial increase in the installation costs.
A further drawback consists in the fact that the structure of the protective container is necessarily more complex since it is necessary to provide at least one inlet opening for the coating material, a device for closing the said opening as well as a sealing system both for the inlet opening and for the connection zone between the two half-shells which generally form the abovementioned protective container.
Moreover, if the filling material should be a thermosetting resin, as is the case in nearly all installation processes, this aspect constitutes a further disadvantage due to the nature itself of this resin. Handling of the latter, in fact, generally requires the use of suitable precautionary measures and a considerable degree of care since said resins are irritants (to the skin, eyes or respiratory tract) and, in some cases, are even toxic.
It is necessary to point out, moreover, that the use of a protective container according to the known art has the further drawback that it requires an operation involving joining together of the two halves forming it, said operation being performed manually and directly on-site, namely within the laying trench and therefore in precarious conditions, with limited freedom of movement. All this results in greater complexity of operation and an inevitable delay in the installation time.
The technical solution described in the document EP-093,617 referred to above goes beyond the traditional concept of a protective device in the form of a container and suggests the use of a device comprising a set of elongated elements and a retractable sleeve to be arranged around the latter.
However this solution, which is based on a concept different from that of the prior art, also has certain drawbacks.
A first drawback, which is particularly significant for the purposes of achieving an acceptable mechanical strength of the joint, consists in the fact that the type of combination of the above-mentioned elongated elements does not allow the formation of a continuous protective layer able to ensure the same level of protection against impacts over the whole external surface of the joint.
In fact, these elements are arranged alongside each other in the longitudinal direction, parallel to the longitudinal direction of the joint, without forming a continuous protective layer on the outside and over the circumference of the joint.
A further disadvantage of the device according to the document EP-093,617 consists in the fact that the elongated elements which form it are made preferably of metallic material or of moulded plastic material.
As mentioned further above with reference to the containers of the known art, the use of a metallic material for protective purposes is disliked since the device is excessively rigid and is unable to dampen the impacts to which it may be subject, transferring practically entirely the impact energy onto the underlying layers.
Moreover, the choice of these materials results in the protective device being particularly heavy, thereby aggravating the working conditions of the technical personnel responsible for joining the cables.
Furthermore, plastic materials in general do not have a high resistance to violent impacts unless special polymer products are used.
A further problem which the solutions of the known art are unable to solve in a satisfactory manner consists in the disposal of the heat which is produced inside a joint following the passage of electric current. In fact, should said heat not be adequately disposed of, a hot point is formed in the distribution system, said hot point consisting of the joint itself. This fact results in an undesirable reduction in the current flow rate inside the cable.
In order to ensure an at least partial disposal of said heat, the solution of the known art relating to a container filled with filling material requires that the thickness of said material should be sufficiently small. However, if this thickness is particularly small, the mechanical strength of the protective coating is inevitably weakened.
The Applicant has therefore established the need to provide a mechanical protection for electrical cable joints which is able to guarantee a high mechanical impact strength, with particular reference to the installation of underground electrical lines, and an optimum disposal of the heat in the joining zone, this protective coating having a reduced thermal resistance, not being affected by particular problems of toxicity and/or handling and not influencing negatively the weight and the thickness of the joint/protective coating assembly.
The Applicant has perceived, moreover, that there is a need to devise a method for protecting joints which can be implemented in a simple manner and with little effort by the operator and which does not require complex operations, resulting in advantages both in terms of the speed of installation and in terms of lower costs.